This invention relates to color image processing. In particular, this invention relates to methods and apparatus for calibrating digital color imaging devices using colorimetric or spectrophotometric techniques. Digital color imaging devices, such as digital color printers and color copiers, have become increasingly popular in recent years. Indeed, while the cost of digital color imaging devices has dropped significantly, the number of hardware devices and software applications that are capable of producing color output that may be printed on such devices has substantially increased. Further, as the output quality and resolution of digital color imaging devices has improved, the number of uses for such devices has further increased.
For example, digital color laser printers and color inkjet printers now are increasingly used as relatively low cost proofing devices for commercial printing presses. Printing a print job on a printing press is a fairly expensive and time-consuming process. As a result, mistakes or errors in the print job are expensive to correct once a press run has commenced. To minimize such costly errors, high quality inkjet printers may be used to provide a proof of the print job before going to press. Ideally, the output of the proofing printer will visually match the output of the press. As a result, the proof output may be used for purposes of approving the print job or making any necessary modifications to the print job before printing the job on the press.
Referring now to FIG. 1, a previously known printing and proofing system is described. Printing system 20 includes commercial printing system 22 and proofing system 24. Commercial printing system 22 includes input device 26, input profile 28, color processing stage 30, press profile 32 and press 34. Input device 26 may be any device that may be used to create and/or store color image 38. For example, input device 26 may be a color scanner, digital camera, computer workstation, computer memory or other similar device.
Color image 38 includes a bitmap array of pixels, with each pixel including multiple colorant values. For example, if input device 26 is a scanner, color image 38 may include pixels expressed as a combination of red, green and blue (“RGB”) colorants. Colorant values typically are represented as multi-bit digital data values. Thus, if eight bits are used for each colorant, the colorant values may range from 0-255. In this regard, 0 corresponds to no colorant, and 255 corresponds to 100% colorant. The colorant values of color image 38 are defined in the device-dependent color space of input device 26.
Input profile 28 includes transformations between the color space of input device 26 and a profile connection space, such as Commission Internationale de l'Eclairage (“CIE”) XYZ, or other similar profile connection space. A profile connection space derived from the XYZ color space is commonly known as the CIE LAB color space, which expresses color values in a rectangular coordinate system, with the L, a, and b values each corresponding to one of the three dimensions in the system. The L-value characterizes the lightness aspect of the region along an axis ranging from black to white, with corresponding values ranging from 0 to 100. The a-value characterizes the color of the region along an axis ranging from green to red, with positive values corresponding to red and negative values corresponding to green. The b-value characterizes the color of the region along an axis ranging from blue to yellow, with positive values corresponding to yellow and negative values corresponding to blue. Together, the a-value and the b-value may be used to express the hue (“H”) and chroma (“C”) of the region:
      H    =                  tan                  -          1                    ⁡              (                  b          a                )                  C    =                            a          2                +                  b          2                    The zero point in the plane defined by the a-values and the b-values corresponds to a neutral gray color having an L-value corresponding to the intersection of the plane with the L-axis.
Input profile 28 typically is produced in accordance with the profile specification of the International Color Consortium (“ICC”), and hence is referred to as an “ICC profile.” An ICC profile generally includes a transform from the profile connection space to the device space (the “forward transform”), and a transform from the device space to the profile connection space (the “backwards transform”). An input profile, however, typically includes only a backwards transform. For example, if input device is an RGB scanner, the backwards transform of input profile 28 may be used to convert device-dependent RGB colorant values to equivalent device-independent LAB colorant values.
Color processing stage 30 optionally may be used to perform various color processing operations in device-independent color space. For example, color processing stage may include software used to perform color editing or other color processing operations. Press profile 32 includes transformations between the color space of press 34 and a profile connection space, and also is typically an ICC profile. Thus, press profile 32 typically includes forward transform 32a and backwards transform 32b. For example, if press 34 is a conventional four-color offset press that uses cyan, yellow, magenta and black (“CMYK”) colorants, forward transform 32a may be used to convert device-independent LAB colorant values to equivalent device-dependent colorant values CMYK1. Press 34 receives CMYK1 colorant values and provides press output 36 on media designed for use by a printing press.
Proofing system 24 includes press profile 32, printer profile 40, calibration stage 42 and proofing printer 46. In particular, backwards transform 32b of press profile 32 may be used to convert device-dependent colorant values CMYK1 to device-independent LAB colorant values. Printer profile 40 is typically an ICC profile, and includes a forward transform between the profile connection space and the color space of proofing printer 46. Accordingly, the forward transform of printer profile 40 is used to convert device-independent LAB colorant values to device-dependent colorant values CMYK2.
Calibration stage 42 typically includes hardware and/or software that: (a) maps calibrated input values to equivalent uncalibrated input values (sometimes referred to as “linearization”); (b) limits the colorant of each channel; and (c) limits the total colorant of all channels. If proofing printer 46 uses multi-shade colorants, calibration stage 42 also may convert single colorant input values to equivalent multi-shade colorant values. The mapping and per-channel colorant limit functions typically are performed using tables that are designed to match the output response of proofing printer 46 to the output response of press 34, and also limit the colorant of each channel. The total colorant limit function is used to limit the total amount of colorant that may be output by proofing printer 46 to avoid negative image artifacts caused by using excessive colorant. Proofing printer 46 may be a digital inkjet printer, such as a CMYK inkjet printer or other similar printer. Proofing printer 46 receives calibrated CMYK colorant values and provides printed output 48 on media designed for use by an inkjet or laser printer.
The process of “calibrating” a printer typically includes determining linearization table values, per-channel colorant limits, a total colorant limit (“TCL”) and, optionally, distribution functions for multi-shade colorants. Referring now to FIG. 2, a previously known printer calibration process 50 is described. Beginning at step 52, a TCL is determined. In a multi-colorant printer, the amount of colorant for each channel typically is specified as a percentage between 0 and 100%. Thus, on a four-color printer, the maximum sum of all colorants that may be specified is 400%, corresponding to 100% on all four channels. If excessive colorant is used, however, undesirable image artifacts may result that produce an unacceptable print. For example, on inkjet printers, excessive colorant may cause bleeding (an undesirable mixing of colorants along a boundary between printed areas of different colorants), cockling (warping or deformation of the receiving material that may occur from using excessive colorant), flaking and smearing. In severe cases, excessive ink may cause the print media to warp so much that it interferes with the mechanical operation of the printer and may damage the printer. Thus, at step 52, a TCL is determined to minimize the effects of excessive colorant.
Previously known techniques for determining a TCL typically rely on trial and error methods that may be unsuitable for proofing purposes. In particular, previously known techniques typically involve printing several color patches that include various combinations of total amounts of colorant. A user then visually inspects the resulting printed output, and selects the patch (and thus the TCL) that produces the “best” results. A problem with such previously known techniques, however, is that the results may vary substantially from user to user, and even from time to time by the same user. The resulting lack of repeatability impairs the goal of obtaining a highly accurate proof.
Referring again to FIG. 2, after determining a TCL, at step 54 a colorant limit is determined for each channel. In a conventional printer, such as a CMYK inkjet or laser printer, the chroma response of the C, M and Y colorants as a function of the colorant amount is quasi-linear. However, beyond a certain specified colorant amount, the chroma actually begins to decrease, and the chroma response becomes highly non-linear. For the K channel, the luminance decreases with increasing colorant amount, until the luminance reaches a minimum level, but further increases in the colorant amount produce no further decrease in luminance. Indeed, for some combinations of colorants and media, oversaturation may occur, in which printed colors do not become any darker, and may actually become lighter, with increasing colorant amounts. Because it is difficult to accurately profile a printer in the non-linear region of operation, previously known techniques for calibrating a printer typically limit the colorant of each channel so that the printer operates only in the quasi-linear region and not in the oversaturation region.
Previously known techniques for determining per-channel colorant limits, however, have typically relied on density-based measurements that may be incomplete and inaccurate for proofing purposes. In particular, previously known techniques for determining per-channel colorant limits typically involve printing a target for each colorant, where the target includes several color patches that range from 0 to 100% colorant. After printing the target, a user typically measures the optical density of each patch using a densitometer or other instrument that provides optical density values. The per-channel colorant limits are then specified as the colorant values that produce a predetermined density (e.g., the lowest maximum density) on all channels.
One problem with such isometric density techniques is that they fail to consider the impact of the colorant limitation on the gray balance of the printer. When a printer outputs approximately equal percentages of C, M and Y colorants, a neutral gray should result. The human eye is very sensitive to detecting shifts in neutrality when neutral areas are compared side-by-side. Thus, gray balance may be used to determine if the gamut of one printing device (e.g., a proofing printer) matches the gamut of another printing device (e.g., a press). Previously known density-based techniques for determining per-channel colorant limits, however, typically do not ensure proper gray balance. To solve this problem, experienced users have developed their own techniques for achieving a desired density value for each colorant and also a good gray balance. Such empirical techniques vary from user to user, however, and require specialized knowledge that all users may not possess.
In addition, previously known density-based techniques for determining per-channel colorant limits may be inaccurate for proofing printers. Conventional densitometers typically operate by illuminating a printed patch using light having a known spectral distribution, and then measuring the amount of light absorbed in a narrow frequency band of the visual spectrum. The absorption measurement may then be translated to a density measurement, with higher absorption corresponding to higher density. Densitometers typically use narrow-band optical filters that are tailored to describe the behavior of colorants used on a conventional printing press. Unfortunately, however, the filters are not optimized for describing the behavior of colorants used by conventional inkjet and laser printers used for proofing. Indeed, if a colorant used by a proofing printer has a maximum absorption at a frequency outside the band of the instrument's filters, the resulting density measurements may be incorrect. As a result, density-based techniques for determining per-channel colorant limits may produce inaccurate results.
Referring again to FIG. 2, after per-channel colorant limits have been determined, in step 56, linearization tables are calculated for each channel so that the output response of proofing printer 46 matches the output response of press 34. Previously known techniques for calculating linearization tables typically involve printing a target for each colorant that includes several color patches that range from 0 to 100% colorant coverage. After printing the target, a user typically measures the optical density of each patch using a densitometer or other instrument that provides optical density values, and then calculates table values that map the input/output density response of the printer to an input/output density response of the press. As described above, however, conventional densitometers and similar measuring instruments may not accurately measure density of colorants used by conventional inkjet and laser printers used for proofing. As a result, previously-known density-based techniques for calculating linearization tables may produce similarly inaccurate results.
Referring again to FIG. 2, after linearization tables have been calculated, at optional step 58, distribution functions may be determined for multi-hue colorants. In particular, high-quality digital inkjet printers used for proofing purposes often include four primary CMYK colorants (also referred to herein as “normal cyan,” “normal magenta,” “normal yellow” and “normal black”), plus light cyan and light magenta colorants (indicated by lowercase “c” and “m”) to provide improved image quality in the highlight regions of an image. Referring again to FIG. 1, if proofing printer 46 is a CcMmYK printer, printer profile 40 typically converts LAB values to CMYK values, and calibration stage 42 converts cyan values into mixtures of normal cyan (C) and light cyan (c) values, and converts the magenta values into mixtures of normal magenta (M) and light magenta (m) values.
Previously known techniques for converting a specified colorant value to equivalent multi-shade colorants often rely on trial and error techniques to determine the distribution function between the colorants. The resulting distribution function may be acceptable for a first set of colorants (e.g., used in a first printer in location A), but may be unacceptable for a second set of colorants (e.g., use in a second printer in location B). As a result, unless a new distribution function is determined for the second set of colorants, the printed output of the two printers may not match. Previously known trial and error techniques, however, typically do not permit easy modification of distribution functions. Instead, the entire process must be repeated, which may be extremely time consuming and inefficient.
In view of the foregoing, it would be desirable to provide apparatus and methods for calibrating a digital imaging device in a repeatable manner.
It also would be desirable to provide apparatus and methods for calibrating a digital imaging device in an accurate manner.
It additionally would be desirable to provide apparatus and methods for calibrating a printer without requiring specialized knowledge by a user.